The invention relates to a method for refining and homogeneously distributing alloying partners and for removing undesired reaction products such as oxides and/or slag in or from soft solder during the production of fine solder powder.
Known from WO 03/051,572 is a lead-free soft solder based on a tin/silver/copper solder alloy in which 0.8 to 1.2 wt. % indium and 0.01 to 0.2 wt. % of an element of the lanthanoids such as for instance lanthanum or neodymium are always alloyed to a base alloy with 5 to 20 wt. % silver, 0.8 to 1.2 wt. % copper, the rest tin and normal impurities.
Beginning at 214° C., this known lead-free soft solder behaves eutectically, suppresses the development of large tin dendrites, ensures a smooth and homogeneous surface after melting, and also has good physical and chemical properties such as for instance very good wettability, high endurance limit under reversed stress, good corrosion resistance, plasticity, and strength, and also a slight electrical resistance.
However, the melting of the alloy and also the processing of this known alloy into fine solder balls leads, in particular due to the very high reactivity of the alloy components lanthanum and neodymium, to serious problems that are exhibited in an agglomeration of reaction products of the neodymium or lanthanum in the solder balls. Neodymium reacts extremely vigorously, for instance even with bound residual oxygen in the melt that is always present in traces, and forms oxides that also agglomerate in the solder balls to larger formations. These oxides accumulate a relatively large amount of neodymium, which is then missing in the matrix or grain boundaries. Furthermore, intermetallic phases with the composition Ag3Sn and Cu6Sn5 occur.
The processability and electrical properties of such soft solders are permanently limited by these hard occlusions.
Known from DE 19 830 057 C2 is also a method for producing without pressure soft solder powder in a grain size from 1 to 100 μm, in which the metallic solder is melted in a high temperature-resistant vegetable or animal oil, then stirred and dispersed in multiple shear treatments by rotors and stators into solder balls of defined size.
In accordance with U.S. Pat. No. 5,411,602 A, solder is melted and the melted solder is divided into drops by means of inert gas. Despite the inert gas atmosphere, it is not possible to prevent reactive metals such as neodymium or lanthanum from reacting with bound oxygen so that the solder alloy contains more or less oxides of these metals and the doping for instance with neodymium or lanthanum is more or less lost.
U.S. Pat. No. 6,231,691 B1 describes a lead-free solder alloy with a tin base with 4.7 wt. % silver, 1.7 wt. % copper, and 0.15 wt. % nickel. This known alloy tends toward increased formation of intermetallic phases of the Cu3Sn and/or Cu6Sn5 type, which have very negative effects on the mechanical/physical properties of the soldering joint.